Wow scales terribly with AMD cpus and prefers Nvidia GPUs. You unfortunately have a bad combination for that game.

FPS in WoW has been terrible and inconsistent since its inception. If you turn everything down you can play it on almost anything, but crank it up and you never know what will happen. And lets face it, the graphics are pretty terrible even on Ultra, it has always been better to just turn down the settings til you get a stable playing experience.

CPU vendor won't make a difference when the situation on screen is tough (25-120 players fighting it all out)-there is gonna be lag no matter what. His best bet is an overclocked i5/i7 (Sandy or newer) but failing that, he is better off with an O/C FX-8320/8350 than some locked i3/i5.

I was having random fps crashes for a long time when trying to run in DX11 so I eventually just stuck with DX9. Then I started just messing with settings to see if I could get DX11 working,turns out changing it so my gpu's handle image scaling instead of letting my monitor do it fixed all my issues.

And fps drops are a fact of life in WoW,I can be running at 120+ in the Vale,as soon as I get near the Shrine my fps craters to 30-40 due to the number of players,I play on Bleeding Hollow so YMMV.

Your Processor is not even twice as good as my old Athlon 64x2 for WoW., I went down to single digits with that old thing. Don't bother looking at other possible causes, I'd bet you a processor upgrade itself that that is what is causing your low FPS in large fights.

Want to test it yourself? Ok. What ELSE could possibly cause it?
RAM? Check the Task Manager to see if you ever go over your limit, and are forced to use the Page File.
GPU? Get a monitoring program, and when you get the FPS drop look at your GPU and CPU Usage. If your CPU Usage on one core is at 100%, and your GPU is low, then it is undeniable.

If you had a better CPU, I'd say it could have possibly been your GPU, but even then that's unlikely. With my 2600K and GTX 550 Ti, I rarely go below 60 FPS but sometimes in big fights, like LFR, World Bosses, and cities I go a little below. In certain small fights like Murozond in End Time, I found that it was extremely heavy on Particle effects, so while I could do LFR with Particles on High, this fight required me to turn them down a bit to prevent low FPS, because one of his spells used a ton of particles.

CPU vendor won't make a difference when the situation on screen is tough (25-120 players fighting it all out)-there is gonna be lag no matter what. His best bet is an overclocked i5/i7 (Sandy or newer) but failing that, he is better off with an O/C FX-8320/8350 than some locked i3/i5.

CPU vendor doesn't matter. It's the CPU itself. It just so happens that the modern CPUs that are on the market are fairly even in overall performance, but Intel has half the cores. So, Intel's cores are roughly twice as fast as AMD cores, and in a game that's single threaded, that means everything. It doesn't mean a lot, it means everything. Those other 7 cores on that new 8-core AMD won't do a damn thing for you unless you plan to run 8 separate WoWs, just like the other 3 on the intel quad core won't help either.

Anything short of a modern High-End Intel CPU will always give you FPS drops in large crowded areas in WoW, because Even the best Intel CPUs aren't immune to FPS drops in WoW, but they'll certainly leave you with the highest possible framerate you can get with what is on the market at the moment.

Rendering a simpler scene will always be quicker than a complex scene, and the difference between a complex and simple scene is how many players are on your screen, along with what they're doing. If you're in a raid with everyone throwing spell effects around and being crowded, then you better expect your 100 FPS to drop down a bit, because your hardware is being stressed.